Test multi-thread merge of multiple profdata files.

RUN: llvm-profdata merge %p/Inputs/multiple-profdata-merge.proftext -o %t
RUN: llvm-profdata merge -j 4 %t %t %t %t -o %t_2
RUN: llvm-profdata show %t_2 | FileCheck %s

; CHECK:Total functions: 11
; CHECK:Maximum function count: 5600000
; CHECK:Maximum internal block count: 2800000


